The present invention relates to a coating method and a coating apparatus, and particularly relates to a coating method and a coating apparatus using an extrusion-type coating head for coating the surface of a running support with a coating composition extruded toward the surface of the support continuously to thereby form a thin film coating on the support with a uniform thickness at a high speed.
Conventionally, various extrusion-type coating apparatuses for coating the surface of a running support with a coating composition extruded onto the surface of the support continuously to thereby form a thin film with a uniform thickness at a high speed have been known, for example, as disclosed in Japanese Patent Unexamined Publications Nos. Sho-57-84771, Sho-58-104666, Sho-59-238179, Sho-63-88080, Sho-63-164022, Hei-2-17971, etc. As a method of sealing an upstream-side edge of a coating head with a pre-coating layer to block off air accompanying a coating layer, there have been disclosed a method of performing pre-coating with a composition having the same components (see Japanese Patent Unexamined Publication No. Sho-58-205561), a method of coating a support with a solvent as a pre-coating (see Japanese Patent Unexamined Publication No. Sho-61-139929) , etc. With such coating methods, it is possible to realize coating with a thin film at a high speed.
When any such coating method is applied to an extrusion-type coating apparatus, a pressed state is produced between a downstream-side edge and a support to thereby smooth a coating layer. As a result, if there is an unevenness in the widthwise direction of the support, the thickness of the coating film is also made uneven in the widthwise direction. Further, if foreign matter is present on the support or mixed in the pre-coating composition, coating layer or coating composition, the foreign matter is apt to be trapped on the downstream-side edge, so that stripes are produced.
For the coating composition, there are available, for example, a photographic photosensitive coating composition, a magnetic coating composition, a surface-protection/charge-prevention or smoothing coating composition, etc. As representative products produced thereby, there are various kinds of photographic film, printing papers, magnetic recording media, etc.
Commonly assigned Japanese Patent Unexamined Publication No. Sho-63-20069 describes a coating apparatus in which it is possible to prevent variations in thickness of a coating film caused by stripe faults or unevenness in support thickness, Young's modulus, etc., and it is further possible to reduce pressure losses when a coating composition passes through a slot.
Further, Japanese Patent Unexamined Publication No. Sho-63-20069 discloses a coating apparatus for forming a coating layer on a support 1 having a coating surface to be coated which is liquid-sealed with an organic solvent 6 applied thereon in advance, using an extrusion-type head having a front edge 2 disposed on the upstream side in the direction of movement of the support, and a back edge 3 disposed on the downstream side in the direction of movement of the support, the back edge 3 having a top end which recedes stepwise than the front edge in the direction away from the support and having an acute-angled top end portion, as shown in FIG. 5.
First, the coating surface of the support is coated with an organic solvent by a conventional coating apparatus such as a gravure coater, a roll coater, a blade coater, an extrusion coater, a rod coater, a wire bar coater, or the like, so that the thus-formed layer prevents air accompanying the front edge upstream side from intruding into the coating layer, thereby to make it possible to maintain a coating state with no faults and to improve high speed coating characteristics.
The front edge 2 is disposed on the upstream side of the support 1 from the exit of the slit 8, and is formed so that the whole area of the edge surface opposite the support 1 bulges toward the support 1. Although a curved surface having a certain curvature is generally used as the shape of the bulge toward the support, the shape is not limited to this, and any shape may be used so long as it can prevent accompanying air from being caught into.
The back edge 3 is disposed so that its top end portion is positioned so as to be away, in the direction opposite to the support, from a tangent drawn to the front edge 2 at the exit portion of the slit 8. Consequently, no pressure force from the support 1 acts against the back edge 3, so that the entrapment of foreign matter at this portion can be prevented. Accordingly, coating surface faults caused by the foreign matters can be reduced.
In an extrusion-type coating head as shown in FIG. 5, however, it has become understood that many stripes can be produced according to the conditions of the surface roughness of a front edge surface 5 and a back edge surface 3b of its top end portion, and the conditions of edge angled portions 9 and 11.
This means that although the front edge surface 5 and the back edge surface 3b in the above-mentioned coating head are ground into required shapes with a high accuracy by using a grinding machine, undesirable conditions may occur in the surface roughness, the straightness of the edge angled portions 9 and 11, etc., in accordance with the conditions at this grinding process, such as the feeding speed, the cutting depth, the selection of grinding tool, or the like, or some material of the coating head top end portion makes conditions, such as the surface roughness, the straightness of the edge angled portions 9 and 11, etc., so that there is a tendency for such undesirable conditions to be manifest on the casting surface directly.
That is, in a conventional coating head having a doctor edge, such as that disclosed in Japanese Patent Unexamined Publication No. Sho-60-238179, or the like, a coating composition is smoothed by the doctor edge (back edge) so that even if the front edge surface 5 and the back edge surface 6 mentioned above are somewhat poor in their surface roughness or have broken portions, the fluid function of the coating composition immediately after application is increased by an internal stress produced by the pressure applied to the coating composition as a result of the above smoothing function.
It could be considered that the finished states of surface roughness of the respective edge surfaces can be compensated by varying the liquid behavior of the coating composition ejected from a slit. However, one would not wish to apply high pressure to the coating composition at the time of coating as shown in FIG. 3, or otherwise coating stripes or unevenness of thickness would be apt to appear to cause deterioration in the quality of the coating surface.
As has been described above, if smoothing by a doctor edge desirably is performed, if rough surface conditions or broken portions exist in a slit inner surface and a front edge surface, not only are faults such as stripes are caused, but also, particularly, unevenness of thickness is produced according to the degree of the straightness of the respective surfaces. Accordingly, not only it is required that there be no surface roughness or broken portions in the slit inner surface and the front edge surface as well as in the back edge surface, but also it is particularly required that the straightness be good in the slit width direction.
It is, however, impossible to make the conditions of the ground or surface-finished state of the coating head high to an unlimited extent. Moreover, the finished state of the coating head has depended to a large extent on the operator's experience and skill.